“I am deeply saddened by the tragedy in Wisconsin and my heart goes out to the families of those affected by this terrible act,” says Eric Knudsen, according to NBC News.

Morgan Geyser and Anissa Weier – who were charged as adults with attempted murder in Waukesha, Wisconsin, on Monday – told police that they had plotted to kill their friend for months so that they could become “proxies” for Slender Man, a character that Knudsen created online in 2009.

Knudsen, a 33-year-old who lives in Florida with his wife and toddler, came up with the character for a thread called “create paranormal images” on the website Something Awful.

The character was soon co-opted by people who created a folklore around the ghostlike figure and made it the focus of online fan fiction and scary YouTube videos.

“I liked the concept of a monster, a creature that causes general unease and terror,” Knudsen said in a podcast interview earlier this year.